Ink printing apparatuses can be used for single-color or multicolor printing to a printing substrate (for example a single sheet or a web-shaped recording medium) made of the most varied materials (paper, for example). The design of such ink printing apparatuses is known; see for example EP 0 788 882 B1. Ink printing apparatuses that operate according to the Drop on Demand (DoD) principle have a print head or multiple print heads with nozzles comprising ink channels, the activators of which nozzles—controlled by a print controller—excite ink droplets in the direction of the printing substrate, which ink droplets are directed towards said printing substrate in order to apply print dots there for a print image. The print image can thereby be made up of image points (what are known as pixels) arranged like a raster. In the following, image points that have been inked by an ink droplet (inked image points) are designated as inked pixels or print points; image points that have not been inked (uninked image points) are called uninked pixels. The activators of the nozzles can generate ink droplets piezoelectrically (DE 697 36 991 T2), for example.
In an ink printing apparatus, the ink that is used is adapted in terms of its physical/chemical composition to the print head; for example the ink is adapted with regard to its viscosity. Given low printer utilization, in the printing process not all nozzles of the print head are activated; many nozzles have downtimes, with the consequence that the ink in the ink channel of these nozzles is not moved. Due to the effect of the evaporation out of the nozzle opening, the danger exists that the viscosity of the ink then changes. This has the result that the ink in the ink channel can no longer move optimally and exit from the nozzle. In extreme cases, the ink in the ink channel dries up completely and blocks the ink channel, such that a printing with this nozzle is no longer possible.
A drying of the ink in the nozzles of a print head during their print pause represents a problem that can be prevented in that a flushing medium (for example ink or cleaning fluid) is flushed through all nozzles within a predetermined cycle. This flushing cycle can be set corresponding to the print utilization.
Furthermore, from DE 697 36 991 T2 (EP 0 788 882 B1) it is known to remedy difficulties caused by the change of the viscosity of the ink in the nozzles upon the ejection of ink droplets in that the piezoelectric activators of the nozzles are respectively vibrated before or after the printing process (also called prefire or meniscus vibrations), such that no ink droplets are ejected but the ink in the nozzles is stirred. It can thereby be achieved that the ink situated at the nozzle openings mixes with the ink located inside the piezoelectric activator, such that the ink droplets can be generated again under normal conditions in the printing operation.
In the printing of a printing substrate it is sometimes necessary to briefly interrupt the printing operation (for example for 3 min), for example in order to monitor the register quality after proofing a print job or in order to correct problems in the post-processing of the printing substrate. The feed speed of the printing substrate can thereby be reduced up to a complete stop in a slowing ramp and be accelerated again in an acceleration ramp after a wait time (of 3 min, for example). During the slowing time period of the printing substrate before the print interruption and the acceleration of the printing substrate after the print interruption, printing can be continued, wherein the time intervals between the print clock pulses (and therefore between the emissions of ink droplets) increase or decrease during the ramps. During the duration of the ramps, the problem of ink drying out in the nozzles of the print heads are then intensified, with the consequence that printing can no longer be conducted sufficiently well.